Ethanol is a major chemical used in human beverages and food, as an industrial chemical, and as a fuel or a fuel additive in reformulated gasoline that is designed to reduce emissions from automobiles. This invention relates mainly to the production of ethanol for use as a chemical or fuel.
D. Verser (U.S. Pat. Nos. 6,509,180 and 7,351,559) discloses a process for producing ethanol that includes a combination of biochemical and synthetic conversions reportedly resulting in high yield ethanol production with concurrent production of co-products.
D. W. Tedder (U.S. Pat. No. 4,517,298) discloses a method of producing alcohol which is at least substantially free of water using a combined fermentation/extraction process.
F. Shinskey (U.S. Pat. Nos. 4,358,346 and 4,502,921) discloses the control of multiple distillation columns for producing anhydrous alcohol suitable for blending with gasoline to produce gasohol.
Ethanol can be derived from carbohydrates to produce ethanol in two basic conversion steps. The first reaction is hydrolysis of the carbohydrates into fermentable sugars, sometimes referred to as saccharification. The second reaction involves the conversion of the sugars to ethanol, commonly done by fermentation with a yeast or some other fermenting micro-organism.
Ethanol produced from the carbohydrates in biomass materials is often referred to as cellulosic ethanol and is usually produced from non-food crops such as agricultural residues (e.g., citrus peel waste (“CPW”), wheat straw, corn stover, bagasse, beet pulp, apple pommace, and corn husks), woody materials (e.g., hurricane debris, sawdust, soft wood, hard wood, and forestry waste), energy crops (e.g., switch grass, canes, and poplar trees) and waste materials like Municipal Solid Waste (“MSW”).
Regardless of the source, efficient utilization of sugars theoretically increases the overall yield of ethanol from biomass and makes production more economically attractive. However, a major limitation of known processes is the complexity and viscosity of the hydrolyzate that results from treatment of the biomass to produce the fermentation medium.
The basic science for converting CPW into ethanol was developed over ten years ago. CPW contains several mono and disaccharides, the main ones being glucose, sucrose and fructose. In addition, CPW contains polysaccharides such as cellulose, hemicellulose and/or pectin (Ting and Deszyck, 1961).
In order to maximize the monosaccharide levels, the polysaccharides present in the CPW may be hydrolyzed. For example, cellulose, hemicellulose and pectin are hydrolyzed using a cocktail of pectinase, cellulase, and beta-glucosidase enzymes to produce glucose, fructose, arabinose, xylose, galactose, rhamnose, and galacturonic acid (GA) (Nishio and Nagai, 1979; Marshall et al., 1985; Ben-Shalom, 1986; Echeverria et al., 1988; Grohmann and Baldwin, 1992; Grohmann et al., 1994, 1995). In turn, the monosaccharides such as fructose, glucose, sucrose, and galactose from citrus waste hydrolysates can be fermented by Saccharomyces cerevisiae yeast (typically used in the brewing industry) to produce ethanol (Grohmann et al., 1994).
Among the citrus waste that has been studied is peel waste from Valencia oranges, the main citrus crop in Florida. The dry matter content observed for peel waste from Valencia oranges is reportedly 24-27% by weight (Ting and Deszyck, 1961; Wilkins et al., 2005). Valencia peel having about 23% dry matter has been indicated to yield sugars for on a % dry matter basis (Grohmann and Baldwin, 1992; Grohmann et al., 1994, 1995) that theoretically provides ethanol in a yield of 6.6% by volume (5.2% by mass) (Grohmann et al., 1994).
Generally speaking, the commercial recovery of alcohol by distillation from fermentation beers has been in widespread operation for many years. Control systems for improving quality within reasonable efficiency limits have paralleled the growth of this industry. In the past, most of the alcohol distilled was for beverage purposes with no crucial requirement for a dehydrated end product, thereby alleviating to some extent both the energy required to distill the alcohol and the need for tight controls over the process. However, in the energy context, rising costs and a need for greater efficiency has focused attention on the need for optimization of energy intensive (endothermic) processes through the application of dynamic control strategies. For example, the production of ethanol from grain blended with gasoline forms the motor fuel “gasohol.” To be effective as an alternative energy source, the process by which the ethanol is produced must minimize energy consumption so as to achieve “a net energy gain” and final stage ethanol should be essentially anhydrous.
Significantly, there are considerable differences between grain-based fermentation beers and cellulosic-based fermentation beers that may affect the “net energy gain.” As a threshold matter, ethanol concentration in the biomass (cellulosic) brews tends to be relatively lower. Moreover, cellulosic fermentation mixtures typically have higher solids levels and/or higher viscosity than their grain-based cousins. Consequently, typical distillation trains that effectively remove ethanol from grain fermentation broths are incapable of handling the higher solids levels and/or increased viscosity present in fermented biomass mixtures. While dilution of the fermentation beer might reduce viscosity or solids related issues, it would further dilute the ethanol concentration. Diluting the beer so as to have the physical characteristics suitable for a traditional beer stripper would result in an ethanol concentration which may be too dilute for economical purification, which reportedly requires an incoming ethanol concentration of at least 3% (Alcohol Textbook 2001)
Accordingly, there is considerable interest in developing new methods for the production of ethanol from biomass that are capable of handling typical viscosities and/or solids levels associated with these feedstocks. There is also interest in developing new methods for the production of ethanol from biomass capable of separating ethanol from ethanol-containing fermented biomass mixtures efficiently and economically, and in some cases without resorting to feedstock dilution to circumvent higher viscosity and/or solids levels. The present invention is directed to these and other important ends.